The present invention generally relates to a color cathode-ray tube and a method for manufacturing the color cathode-ray tube. More specifically, the present invention is directed to provide such a color cathode-ray tube and a manufacturing method thereof capable of reducing light/dark line patterns produced while the color cathode-ray tube is operated.
Normally, a fluorescent screen is formed on an inner surface of a face panel of a color cathode-ray tube, and the fluorescent screen owns three color fluorescent layers made of three fluorescent layers on which a large number of dots, stripes and the like are formed and emit red, green, blue light. This fluorescent screen is manufactured by using the photographic printing method involving light exposing/developing steps. In other words, a photosensitive film is coated on the inner surface of the face panel, this coated photosensitive film is masked by way of a shadow mask, and then exposing light is irradiated onto the inner surface of the face panel. As a result, the exposing light which has passed through apertures (light passing holes) of the shadow mask may expose the photosensitive film to thereby form the above-described dots, stripes and the like.
On the other hand, paths of electron beams while a cathode-ray tube is operated are different from those of exposing light during an exposition operation. As a consequence, conventionally, in order to improve the beam landing characteristic, the following method has been introduced. That is, a correction lens is arranged in an exposure optical system, and exposing light is refracted by this correction lens in the exposing step, so that this exposing light is approximated to the actual orbit of the electron beams while the cathode-ray tube is operated.
The conventional method for forming the fluorescent film of the color cathode-ray tube with employment of this sort of correction lens is disclosed in, for instance, JP-A-47-40983 published in 1972. Referring now to drawings, the fluorescent film forming method of the color cathode-ray tube disclosed in JP-A-47-40983 will be explained.
FIG. 1 schematically represents a structure of a light exposure base used to manufacture a fluorescent film of a color cathode-ray tube according to the prior art.
Within the light exposure base 84 shown in FIG. 1, there are built the constructive elements such as a light exposing light source 81, a correction lens 82 having a continuous curved surface (will also be referred to as a "continuous lens" hereinafter), and another correction lens 83 having a discontinuous curved surface (will also be referred to as a "discontinuous lens" hereinafter) in this order from a bottom of this light exposure base 84. An opening portion 88 used to pass therethrough the exposing light emitted from the light exposing light source 81 is formed in an upper surface of the light exposure base 84. A face panel 85 on which a shadow mask 87 is mounted on the side of an inner surface thereof is mounted on the light exposure base 84. It should be noted that a photosensitive film 86 is coated on the inner surface of the face panel 85.
In the light exposure base 84 having the above-described arrangement, the exposing light projected from the exposing light source 81 is refracted by the continuous lens 82 and the discontinuous lens 83, and then passes through the apertures of the shadow mask 87 to reach the inner surface of the face panel 85, so that the photosensitive film 86 coated on the inner surface of the face 85 is exposed.
In this case, the discontinuous lens 83 provided inside the light exposure base 84 has such a function to refract the exposing light projected from the exposing light source 81, and also approximate the optical path of the exposing light to the orbit of the electron beams of the cathode-ray tube. As a consequence, the discontinuous lens 83 owns a very complex shape. FIG. 2 represents one example of the discontinuous lens 83. FIG. 2A is a front view for schematically showing this discontinuous lens, FIG. 2B is a sectional view for indicating the discontinuous lens shown in FIG. 2A, taken along a line A--A of FIG. 2A, and FIG. 2C is a sectional view for denoting the discontinuous lens shown in FIG. 2A, taken along a line B--B of FIG. 2A. As indicated in FIG. 2A to FIG. 2C, the discontinuous lens 83 is constructed in such a manner that a plurality of light incident surfaces 83a are arranged in a matrix form, and these light incident surfaces 83a own inclinations along a height (z) direction with respect to a horizontal (x) axis and a vertical (y) axis. Then, a level difference plane 83b is formed at a right angle to a light projection plane 83c at a boundary between the adjoining light incident surface 83a.
FIG. 3 is a perspective view for showing a mold used to form the discontinuous lens 83 shown in FIG. 2A to FIG. 2C. The mold used to form the conventional discontinuous lens is a so-called "assembled type mold", and as indicated in FIG. 3, is constituted by assembling a plurality of blocks 123 having planes 123a corresponding to the respective light incident (incoming) planes 83a of the discontinuous lens 83.
As previously described, in the above-explained discontinuous lens 83, the level difference plane 83b between the adjoining light incident planes 83a is provided at a right angle with respect to the light projection plane 83c. As indicated in FIG. 3, this is because a level difference 123b between the respective blocks is set perpendicular to a rear surface 123c of the mold in order that the respective blocks 123 for constituting the assembled type mold are assembled with each other without any space. The conventional discontinuous lens 83 owns the following problems since the level difference plane 83b is located at a right angle with respect to the light projection plane 83c.
FIG. 4 is a view for partially enlarging the discontinuous lens 83 shown in FIG. 2. The exposing light which is emitted from the exposing light source 81, passes through the continuous lens 82, and then is entered into the light incident (incoming) plane 83a of the discontinuous lens 83, is refracted at this light incident plane 83a. Thereafter, the refracted exposing light reaches the light projection (outgoing) plane 83d of the discontinuous lens 83. Then, this exposing light is refracted at the light projection plane 83 to be projected outside the lens, and is irradiated onto the inner surface of the face panel 85. However, the exposing light directed to a portion (portion "A" of FIG. 4) near the bottom of the light incident plane 83a will be entered into another portion (portion "B" of FIG. 4) near a summit portion of the adjoining light incident plane 83a. This exposing light entered into the portion near the summit portion is refracted at the light incident plane 83a, and thereafter would be refracted/reflected at the level difference plane 83b between the adjoining light incident planes 83a. As a consequence, the portion near the bottom portion of the light incident plane 83a could not be effectively utilized and such a region will be produced that the luminous flux density of the exposing light projected from the discontinuous lens 83 is lowered by a width "t" corresponding to a height of the level difference plane 83b. As a result such a portion is formed in the photosensitive film 86 coated on the inner surface of the face panel 85. That is, in this portion, the insufficient exposing process is carried out with the width "t" corresponding to the height of the level difference plane 83b of the discontinuous lens 83. This insufficient exposed portion will appear as the dark line pattern when the cathode-ray tube with employment of this face panel 85 is operated. As a result a shadow and a partially dropped display screen will be displayed, so that it is rather difficult to obtain a color cathode-ray tube with a high definition and a high image quality.
Also, since a plurality of blocks 123 having the planes 123a corresponding to the respective light incident planes 83a of the discontinuous lens 83 are assembled to each other so as to construct the mold for forming the above-explained discontinuous lens, there is a limitation when the area of the plane 123a and the height of the level difference 123b are made small, taking account of the precision and the like defined when the respective blocks 123 are combined with each other. As a consequence, it is difficult to reduce the area of the light incident plane 83a and also the height of the level difference plane 83b in the discontinuous lens 83 formed by using this mold. It is conceivable that the pitch of the light incident plane 83a is limited to on the order of 8 mm in the conventional discontinuous lens 83 formed by using the assembled type mold. This fact could not give a satisfactory solution as to needs for high-definition color cathode-ray tubes. That is, in this high-definition color cathode-ray tube, the screen which has been conventionally constituted by 400,000 pixels is tried to be constituted by pixels more than 1,000,000 pixels.